With regard to aircraft structures, torsion boxes are typically applied to stabilize large elements such as horizontal and vertical tailplanes or airfoils while ensuring a low weight of these components. Typically, torsion boxes contain a stabilizing structure consisting of ribs and spars which are arranged in a generally crosswise or net-shaped manner to support thin-walled sheets or skins which are attached to a top and bottom surface of this net-shaped arrangement. The sheets or skins may further be stiffened by stringers. To the torsion box, usually a separate leading edge and trailing edge structure, a root joint and a tip are attached to complete the tailplane or airfoil.
In this connection, the ribs and spars are typically longitudinally shaped and comprise a substantially planar main section. The ribs and spars may, for example, be designed with a T-, I- or C-shaped cross-section, wherein the planar main section configures the web section and may be reinforced with stiffeners. The planar main section may generally be designed as a thin element, for example with a thickness of a few centimeters or millimeters. In case of a tailplane or airfoil, the spars are typically arranged to extend transverse to a main longitudinal axis of the aircraft which coincides with a forward flight direction. Accordingly, the spars define a leading and trailing edge of the torsion box with regard to said flight direction. Further spars may be arranged in between these leading and trailing edges and may extend substantially in parallel thereto. The ribs, on the other hand, usually run approximately in parallel to said longitudinal axis of the aircraft and are arranged to connect the spars with each other.
In order to meet weight requirements, it is well-known to use lay-ups consisting of single plies of fiber-reinforced composite materials for designing such ribs and spars. A preferred composite material, in particular for designing vertical tailplane components, is CFRP (carbon fibre reinforced plastic) made of thin plies (0.1 to 0.25 mm) which are stacked to a required thickness. In case of plies with a substantially unidirectional fiber orientation, it is known to specifically select the orientation of the single plies relative to each other and relative to expected main loads on the structural element. By doing so, a specific strength and/or stiffness with regard to specific load scenarios can be achieved.
For a composite structural element of the above-specified type, a coordinate system can be defined with a first axis extending along the longitudinal axis of this structural element and a second axis extending perpendicular to the longitudinal axis within the planar main section and defining an angle of 90° with the first axis. For designing such structural elements out of fiber-reinforced composite materials, so far lay-ups have been applied containing at least one pair of plies arranged in said lay-up such that their directions of fiber orientation extend at an angle of +/−45° in said coordinate system. Similarly, lay-ups containing at least one pair of plies with a fiber orientation of +/−60° in said coordinate system are known. However, the known lay-ups possess an undesirably high weight which results, amongst others, in higher fuel costs when operating the aircraft.
Accordingly, an embodiment provides a composite structural element of the above-mentioned type which possesses a high stability and in particular stiffness at a low overall weight.
A composite structural element, in particular a rib or a spar, specifically for use in a torsion box of an aircraft structural component such as a vertical tailplane is provided. The structural element comprises a substantially planar main section defining a coordinate system with a first axis extending along the longitudinal axis of the structural element and a second axis extending perpendicular to the longitudinal axis within the planar main section and defining an angle of +90° with the first axis. The structural element contains a lay-up of single plies consisting of a fiber-reinforced composite material with a substantially unidirectional fiber orientation. The lay-up comprises at least one symmetrically arranged pair of a first and a second ply which are arranged in the lay-up such that the direction of fiber orientation extends in the coordinate system at an angle in the range of −17° to −23° for the first ply and +37° to +43° for the second ply.
The second axis may generally extend in a direction of or parallel to a main load acting on the structural element, wherein the main load acts transversely with respect to the longitudinal axis. The first axis may be chosen to define an angle of 0°. Furthermore, the composite structural element may be designed within an overall longitudinal shape, thus defining the position and orientation of the longitudinal axis in accordance with the shape.
For the present embodiment, the structural element contains at least one planar main section with the lay-up as specified above, wherein this planar main section may in particular represent the major load-carrying portion of the structural element. Furthermore, this planar main section may specifically be designed to be free of bends or curvatures.
As is well known in the field of fiber-reinforced composite materials, a symmetrical arrangement of single plies within a lay-up describes a symmetrical stacking or sequence of these plies with regard to a geometric center plane of the lay-up. Furthermore, a unidirectional fiber orientation generally expresses an orientation of the fibers such that they run substantially in one common direction.
The structural element according to the embodiment contains a lay-up which distinguishes itself in particular in terms of the fiber orientation of the single plies arranged therein. With regard to the previously described conventional lay-ups, it has been determined that shear forces represent a major load that the composite structural elements, such as ribs and spars, are exposed to. A mode of failure which may arise due to these shear forces is buckling.
The known lay-ups as described above are, however, mainly designed to withstand only one of these main shear forces typically defined as positive shear forces resulting from a so-called direct loading. With regard to such positive shear forces, the conventional lay-ups generally provide a reliable stability and in particular sufficient buckling resistance. Yet, with regard to shear loads in opposite directions, also called negative shear forces or opposite loading, the conventional lay-ups provide a comparably low buckling resistance. As a consequence, the lay-ups have to be built of a larger number of single plies and/or plies having a greater thickness.
By arranging the single plies within a lay-up according to the embodiment, a composite structural element containing this lay-up possesses much higher stiffness when compared to the conventional lay-ups. This relates in particular to an improved resistance with regard to shear forces of direct and opposite loading directions leading to positive and negative shearflows. In fact, it has been discovered that a structural element according to the embodiment possesses an almost equal buckling resistance with regard to both of these types of shear forces.
Accordingly, it is not necessary to compensate for a lower buckling resistance of the lay-up with regard to opposite shear loading by adding further plies and/or generally increasing the thickness of the lay-up. Instead, with a lay-up according to the embodiment, a good compromise between a buckling resistance in both directions of shear forces and the required amount and/or thickness of plies is reached. Therefore, an overall weight reduction of the structural element is achieved.
In the specific lay-up according to the embodiment, a nearly equal buckling resistance to both main types of shear loads may be achieved for a large variety of fiber-reinforced composite materials and over a large range of thicknesses of these materials. In other words, the lay-up according to the embodiment provides a preferable buckling resistance almost independently of the selected material and/or its thickness.
In a preferred embodiment, the first ply is arranged in the lay-up such that the direction of fiber orientation extends in the coordinate system at an angle in the range of −18° to −22°. In a further preferred embodiment, this angular range is −19° to −21°. Likewise, in a preferred embodiment, the second ply is arranged in the lay-up such that the direction of fiber orientation extends in the coordinate system at an angle in the range of +38° to +42°. In a further preferred embodiment, this angular range is +39° to +41°. It will be understood, that the above described preferred embodiments with regard to the angular ranges for arranging the first and second ply may be combined arbitrarily with each other to form a lay-up for a structural element according to the embodiment.
In a preferred embodiment, the direction of fiber orientation of the first ply extends in the coordinate system at an angle of approximately −20°. In this context, the term “approximately” generally denotes a tolerance of +/−1° with regard to the direction of fiber orientation.
In another preferred embodiment, the direction of fiber orientation of the second ply extends in said coordinate system at an angle of approximately +40°. Similar to the above, the term “approximately” denotes a tolerance of +/−1° with regard to the direction of fiber orientation.
The first ply may form an outermost layer of the lay-up. It has been found that by arranging the first ply in this way, the buckling resistance of the lay-up can even further be increased. The lay-up may constitute at least part of a web of the structural element. The web may be configured by the planar main section of the structural element which is typically exposed to particularly high shear forces. Flange sections may be attached to this web to e.g. define T-, I- or C-shaped cross-sections of the structural element. By placing the lay-up according to the embodiment in the web, the overall stability of the structural element can be increased considerably. The structural element may be configured as a unitary member comprising the lay-up. This is in particular advantageous with regard to the manufacturing process of the structural element and avoids further steps for joining single pieces or introducing additional fastening means.
In a preferred embodiment, the lay-up comprises at least one symmetrically arranged pair of first and second plies. In other words, the lay-up does not contain any further plies with a different direction of fiber orientation in the coordinate system. By doing so, a minimum number of plies for ensuring the required stability and, in particular, buckling resistance can be used for building up the lay-up, thus keeping the weight to a minimum.
As an alternative, the lay-up may further comprise at least one ply with a direction of fiber orientation extending in the coordinate system at an angle in the range of −3° to +3°. In a further preferred embodiment, this angular range amounts to −2° to +2°. In a further preferred embodiment, this angular range amounts to −1° to +1°. In a highly preferred embodiment, the respective angle amounts to 0°.
Likewise, the lay-up may further comprise at least one ply with a direction of fiber orientation extending in the coordinate system at an angle in the range of +87° to +93°. In a further preferred embodiment, this angular range is to +88° to +92°. In a further preferred embodiment, this angular range is +89° to +91°. In a highly preferred embodiment the respective angle is +90°.
The above-described further plies may be provided for rendering the lay-up according to the embodiment more stable during its production as well as for generally improving its bearing characteristics. These plies may also be arranged symmetrically within the lay-up.
The structural element may comprise at least two stiffeners extending substantially in a direction of +90° in the coordinate system along the planar main section of the structural element. In other words, the stiffeners may extend substantially transversely with respect to the longitudinal axis of the structural element. The stiffeners may be configured as specifically shaped sections of the structural element, for example, in form of beads or corrugations. Alternatively, the stiffeners may, for example, take the form of additional rigid elements attached to an outside of the structural element. In this way, forces acting in a direction transverse to the longitudinal axis of the structural element can be better compensated for. The stiffeners and the bending stiffness and pitch between them may be chosen in such a way that under shear loading, the local buckling modes between two stiffeners appear prior to global buckling modes.
The area between the two stiffeners may have an aspect ratio as defined by the ratio of its longest and shortest extension of not less than 1.5. On the other hand, the area between the two stiffeners may have an aspect ratio as defined by the ratio of its longest and shortest extension of not more than 4.0. It has been found that the structural element according to the embodiment possesses a particularly high stability when staying above respectively below these threshold values.
In a preferred embodiment, the lay-up does not comprise more than 32 plies in total. This ensures that the structural element containing the lay-up remains generally thin and may be reduced in weight. Likewise, in a preferred embodiment, the lay-up does not exceed a thickness of 8 mm. It may equally be provided that only the thickness resulting from a summation of the individual thicknesses of the first and second plies does not exceed an amount of 8 mm. In this case any further plies, e.g. with a fibre orientation of 0° or +90°, are not considered for determining the respective thickness.
The embodiment furthermore relates to a torsion box for an aircraft structural component, in particular a vertical tailplane, comprising at least one composite structural element according to any of the previously discussed embodiments.